1. Field of the Invention
This invention relates to low NOx emission burners and processes.
2. Description of the Related Art
Prior to combustion system emissions regulations, burners for boilers, fuirnaces, etc., were designed to produce stable and quiet flames that had good heat transfer to the load. The boiler or furnace was only designed to have a combustion volume large enough to allow the flame to completely burn out, without any substantial quenching or instability. It was then left to the burner designer to package the flame in the confines of the boiler or furnace. Since cost was the major factor driving boiler or furnace designs, in many cases the combustion volumes were small, heat transfer was limited and gas temperatures were high. This drove very high NOx emissions, which prompted the introduction of regulations to limit NOx and associated air quality impacts. These regulations have forced the introduction of new burner designs that hopefully can meet NOx emissions targets as well as all other burner performance criteria. However, progress has been limited because burner designers have been reluctant to move away from basic designs that were evolved in a period prior to the imposition of NOx regulations. Significant departures from past burner designs are needed to fully meet all current burner performance requirements.
For safety, conventional burners were designed to produce nonpremixed flames, which generated stratified fuel concentrations and high temperature flame sheets that stretched across the boiler volume. In this type of flame, prompt NOx was produced on the fuel rich side of the flame and thermal NOx was produced within the high temperature flame sheet, where oxygen is available. NOx was initially reduced by controlling flame temperature, through modifications in fuel/air mixing design and/or simple introduction of diluents (e.g. cooled flue gas, steam) into the combustion air. Using diluents primarily reduced thermal NOx, with some impact on prompt NOx, which is less sensitive to temperature than thermal NOx. While NOx was controlled to some extent, burner performance characteristics, like flame stability, CO and unburned hydrocarbon emissions, heat transfer, combustion noise, and turndown, were adversely affected. This occurred even for moderate reductions in NOx. This emissions control approach was ineffective because nonpremixed flame conditions needed for optimal NOx control were in conflict with conditions needed for optimal stability, CO, unburned hydrocarbon emissions, heat transfer, combustion noise and turndown. Therefore, reliance on a single flame zone and flame type in conventional burner design constrained burner performance to be less than optimal in all categories. As NOx emission limits were further lowered, the problem became even more severe, prompting the introduction of expensive post combustion NOx control systems. In this approach a chemical NOx reducing agent is introduced into a downstream reactor, that in some cases contains a catalyst bed to facilitate the NOx reduction reaction. While these systems could reduce NOx to needed levels, and allow simple conventional burners, to be utilized, they substantially increased costs. This is partially because they needed a separate downstream reactor in which to inject a NOx reductant. In addition, with conventional burners, the amount and cost of NOx reduction agent was high, because of the stoichiometric need of the agent versus the initial high NOx.
Regulations now require that NOx be controlled to less than 5 ppm in stringently controlled regions. Given the very substantial expense of post combustion NOx control, there have been recent attempts to modify burners and flames to yield low NOx at costs below that of post combustion NOx control. This has to be accomplished with good flame stability and turndown, low noise and low CO and unburned hydrocarbon emissions. Using specially designed burners, without any post combustion NOx control features, has only been partially successful in achieving these goals.
In U.S. Pat. No. 5,603,906, a burner is described that uses cooled recirculated flue gas, inducted by fuel jets, to cool the initial flame zone and reduce NOx. Using this approach, plus multiple air injection stages stretches out the burning and reduces thermal NOx. This NOx is formed by the oxidation of nitrogen in the air, primarily under high temperature conditions with oxygen present. However, while the flame zones are cooled with this approach, and thermal NOx is reduced, there are still stratified fuel zones where fuel rich conditions yield prompt NOx. In contrast to thermal NOx, prompt NOx is initiated by the reaction of hydrocarbon fuel fragments with molecular nitrogen from the air. The intermediate nitrogenous species that are formed from this reaction (e.g. NH, NH2, NH3, HCN, CN) then have the potential to be oxidized and converted to NOx, once they are contacted with oxygen from the combustion air. Unfortunately, this process can even occur at fairly low temperatures, relative to typical thermal NOx processes. Therefore, to achieve ultra reduced NOx conditions with this type of burner, sufficient cooled flue gas has to be recirculated to suppress the temperature to a level where flame stability is poor. Also, under these conditions, CO and unburned hydrocarbons can be excessive, which is unacceptable. Therefore, these burners are more low NOx than ultra reduced NOx burners. Nonpremixed stratified flame regions are also the limiting condition on NOx reduction in the burner concepts described in U.S. Pat. Nos. 5,542,840, 5,460,512, 5,284,438, 5,259,755 and 5,257,927. In these cases, nonpremixed fuel is injected into the flame zone and ignited, giving stratified fuel regions, where prompt NOx can be formed. By controlling flame zone temperature with cooled flue gas injected into the fuel and/or air, or using other diluents, such as water, steam, etc., thermal NOx can be suppressed. However, ultra reduced NOx emissions cannot be achieved at acceptable flame stability, noise, CO and unburned hydrocarbon emissions levels. For example, the burner described in U.S. Pat. No. 5,460,512 requires over 40% flue gas recirculation to achieve low NOx. At this level, flame stability becomes a problem, and operational safety is of sufficient concern to lead the burner company to recommend further development and testing of this concept.
Recognizing this NOx control limitation of nonpremixed flames and stratified flame regions, attempts have been made to apply premixed flame concepts to low NOx burners. If the burner design uses premixed fuel and air with an excess of oxygen, then stratified fuel regions and prompt NOx processes are minimized. To achieve the needed thermal NOx reduction, burners with premixed or near premixed flames that utilize Flue Gas Recirculation (FGR) dilution have been conceptualized. By premixing the fuel and air with an excess of oxygen, prompt NOx, that is formed in fuel rich regions is suppressed. When combined with flue gas recirculation, that reduces thermal NOx emissions, lower levels of NOx can be achieved. An example of this approach is the burner highlighted in U.S. Pat. No. 5,407,347, which uses swirl vane injection of gas fuel to get a premixed or near premixed state while avoiding flame flashback problems, inherent in premixed systems with near stoichiometric conditions. Combining this burner design with cooled flue gas recirculation can reduce NOx. However, premixed flames are very compact and the flame can easily couple with boiler acoustics, giving excessive noise and even damaging vibrations. In addition, to achieve the ultra-low NOx, very high levels of FGR are needed. According to the patent, 50 to 60% flue gas recirculation is needed to minimize NOx. This can destabilize flames, limiting burner turndown and possibly even leading to flame blowout. If this occurs, the boiler can fill with an unburned mixture of air and fuel, which can then be a severe explosion hazard. Lastly, with large amounts of flue gas, the flame becomes transparent, and heat transfer is reduced in the initial boiler volume to the point where xe2x80x9cback passxe2x80x9d and tube sheet temperatures may exceed boiler design limits. In this case, the boiler could be degraded or even fail. Therefore, conditions that improve NOx with this flame type lead to degradation in other important burner and system performance characteristics. Again, a single flame type constrains performance so that an optimization of all burner performance parameters cannot be achieved.
To address fuel lean low NOx flame stabilization, burner designers introduced fuel rich flames that help stabilize fuel lean flames. Burner designs considering alternating fuel rich and fuel lean premixed flames were proposed in U.S. Pat. Nos. 5,368,476 and 5,073,106. These designs were directed at small capacity applications, such as home appliance burners. By introducing a fuel rich flame adjacent to a mixture of fuel gas and an excess of air, the lean low NOx flame is better stabilized and turndown is improved. However, the prompt NOx in the fuel rich stabilization flame is not addressed in this design. This will tend to offset the NOx reduction achieved by the lean flame and prevent ultra reduced NOx emissions from being achieved. Also, when the fuel rich flame products merge with the fuel lean flame products, the resulting reaction will increase the local burnout zone temperature and tend to produce NOx. Therefore, without some control of the burnout zone temperature, through heat extraction or dilution of the gases, NOx emissions are further compromised. Lastly, although fuel rich flames are more stable then very fuel lean flames, they are less stable than stoichiometric flames. Therefore, adding fuel rich flames to fuel lean flames is not totally satisfactory. To address the stabilization limitation of the designs referenced in the above patents, the burner design referenced in U.S. Pat. No. 5,022,849 proposes using a near stoichiometric stabilization flame interspaced between the fuel rich and fuel lean flames, or attached to separate fuel rich or fuel lean flames at their periphery. As noted above, whether fuel rich or fuel lean, premixed flame stability is compromised. By inserting a near stoichiometric flame between fuel rich or fuel lean flames, or at their periphery, flame stabilization can be improved. However, the improvement is limited. As is well known, premixed flames have a distinct flame speed that must be balanced against the reactant velocity. Unless the premixed flame is anchored to the burner by substantial recirculation of hot products that continually ignite the reactants, flame instability, and blowoff are possible. In addition, the near stoichiometric premixed stabilization flame has a high potential for acoustic coupling and noise generation. Therefore, while flame stability is improved by the design proposed in U.S. Pat. No. 5,022,849, it is not improved to the needed level for stable, low noise and safe operation at ultra reduced NOx conditions. Also, while the patent highlights a low NOx aspect of the design, ultra reduced NOx levels will be difficult to achieve at good flame quality. This is because of fuel rich flame prompt NOx production, near stoichiometric flame high thermal NOx production and higher then desired NOx production in the burnout flame, as a result of the high temperature and lack of temperature control of the burnout flame. Therefore, while an improvement, the design proposed in U.S. Pat. No. 5,022,849, does not optimize stability or NOx emissions.
To better optimize fuel rich flame stability, the design highlighted in U.S. Pat. No. 6,089,855 utilizes a vortex precombustor to contain the burning fuel rich mixture and recirculate combustion products back towards the reactant entrance to continuously ignite and stabilize the flame. Fuel rich flame stability can be improved by recirculating combustion products to continuously ignite and stabilize reactants. Air is then injected downstream to complete combustion. By thoroughly mixing gases within the rich zone, ignition and stability are improved over the open flame burner approaches highlighted in U.S. Pat. Nos. 5,368,476, 5,073,106 and 5,022,849. When air is injected into the fuel rich products downstream of the vortex reactor, any remaining fuel components are oxidized at high temperature. To suppress NOx, final burnout one flame temperature must be reduced. This is indirectly accomplished in the design in U.S. Pat. No. 6,089,855 by the recirculation of cooled products in the fuel rich reactor. This initially lowers the reactor temperature and the burnout flame temperature, which is needed for good NOx control. Furthermore, mixing of the burnout combustion air with cooled combustion products in the furnace is also promoted to control burnout temperature and NOx. While cooling the initial fuel rich reaction helps lower Nox in the final burnout zone, it also reduces the stability of fuel rich flames. Also, while the use of the reactor confinement reduces stratified fuel regions where prompt NOx is formed, the lower temperature and thorough mixing does not recognize important other NOx control benefits of a fuel rich reactor. These benefits are achieved by following the stirred or mixed zone that promotes flame stability by a zone where the rich products are held at high temperature in a plug flow state for a given residence time that causes a suppression of fixed nitrogen compounds as well as NOx. If combined with products from fuel lean flames and reacted together, these fuel rich conditions lead to a minimization of NOx exiting the burnout zone.
Previously defined nonpremixed or premixed low NOx burner flames cannot optimally reduce NOx production, while simultaneously optimizing other important burner performance characteristics. Furthermore, whether nonpremixed or premixed, low NOx burners have focused on suppressing NOx production, primarily by temperature control. However, to minimize NOx, reduction processes, as well as NOx production suppression processes must be considered. By optimizing both processes, NOx can be minimized, while maintaining all other important burner performance characteristics.
The invention in summary provides burner system and processes which bring both NOx production suppression and NOx reduction control functions together in a single design that generates several reaction zones in which NOx control is balanced with other important combustion characteristics such as flame stability, low CO and unburned hydrocarbon emissions, noise, vibrations, turndown, operability and heat transfer.